The copolymerization of ethylene with esters of vinyl alcohol in the absence of substantial amounts of a solvent or emulsifier is known. Thus, according to a process described in DE 21 02 469 Cl, the reaction of the monomers is carried out at pressures of up to 7,000 atmospheres, preferably between 1,500 to 3,000 atmospheres. The polymerization temperatures lie between 150.degree. and 350.degree. C., preferably between 200.degree. C. and 325.degree. C. Oxygen or compounds which decompose to give free radicals under the reaction conditions are used as polymerization initiators. A particular feature of this process is that it is carried out substantially in the absence of solvents or emulsifiers. As a rule, only negligible amounts of an inert liquid which serves as a solvent for the polymerization initiators are added to the reaction mixture. If oxygen is used as a catalyst, no solvents are required.
The very low maximum conversions which are achieved when the reaction mixture passes through the reaction zone once are a disadvantage of this known process. In the copolymerization of ethylene and vinyl acetate in an autoclave with stirring they are only about 20%. In industrial plants, the uncovered monomers are therefore recycled to the reactor.
The decisive factor for the level of conversion is the difference between the polymerization temperature and the temperature of the reaction mixture at the reactor inlet. At this temperature difference increases, the conversion increases or, expressed otherwise, the conversion depends directly on the polymerization temperature at constant reactor inlet temperature.
However, the amount of temperature increase in the polymerization reactor is subject limitations. Above a certain polymerization temperature, the reaction can no longer be controlled. This is due to thermal degradation of the polymer. Moreover, at high temperatures, ethylene may undergo spontaneous decompositions with formation of carbon, hydrogen, and also methane. Thus, in the processes of the prior art, in contrast to the wide temperature ranges of 150.degree. C. to 350.degree. C., a practical upper temperature limit of 300.degree. C. is usually not exceeded in the polymerization of ethylene. The reaction temperature for the copolymerization of ethylene and vinyl esters is substantially lower and is dependent on the specific conditions in the polymerization apparatus. Thus, the maximum reaction temperature in the Example of the cited DE 21 02 469 (in the copolymerization of ethylene and lauryl acrylate) is 280.degree. C., and only 260.degree. C. in the copolymerization of ethylene and vinyl acetate.